This invention relates to the construction of a packaged electronic circuit comprising a molded-plastic support base having a capacity to accept and hold electronic devices or subassemblies thereof in a pocket within the molded substrate, and positioning them for interconnection (hereinafter referred to as xe2x80x9cMolded Electronic Packagexe2x80x9d). The connection to the electronic devices or subassemblies is intricately formed with the placement of the circuit traces on the substrate. This is an advantage over existing technology because it offers savings both in cost and space. This is usually done at the surface level. The formation of the circuit traces by printing with Polymer Thick Film and the attachment to electronic devices is achieved by Polymer Thick Film printing technology and the substrate is formed with plastic molding technology. The electronic devices may be semiconductors, integrated circuits, electromechancial devices, other active components, passive components such as Thick Film resistors or capacitors, or other devices as defined later. While molded substrates are not new, and the use of Polymer Thick Films technology and Thick Film Technology are not new, the combination of a molded substrate with a pocket built into the molded substrate and interconnecting the electronic devices in the pocket with Polymer Thick Film technology is new and fulfills a long-felt need to be able to reserve the surface area above the trace area for other circuits traces and electronic devices. Others have tried to accomplish this by other means of interconnecting by layering circuit boards as discussed below, but only with the advent of the new Polymer Thick Films and the new molded plastic resins which have only recently become available can we now accomplish the connecting of the electronic device in the pocket of the substrate material. The pocket permits the electronic device to be supported by the substrate instead of being supported on the trace which allows for the use of the new Polymer Thick Film technology to connect the electronic devices which previously did not exist. This long felt need to further reduce the size of circuit boards for ever smaller products while containing or reducing costs of the resulting circuits has until now been unanswered by conventional methods.
A traditional printed circuit board comprises a supporting substrate and copper-foil circuit traces. These traces are usually formed by the chemical etching of a pattern defined onto a laminated copper surface. Sometimes both front and back sides of the substrate carry circuit traces. Two-sided, or double-sided designs usually are interconnected through vias (holes) that have copper deposited around the hole walls. A related technology exists known as Thick Film. Here the supporting substrate comprises flat, thin pieces of alumina (Al2O3) on which the traces are printed with an ink containing metal, glass frit, and other additives. When fired at the correct temperatures the ink fuses to form conductive traces to which components can be soldered. An important feature to Thick Film technology is that conductive traces can be interconnected by printed inks having specific electrical resistivity after being heated in a kiln (firing).
A lessor-known technology exists known as xe2x80x9cPolymer Thick Filmxe2x80x9d wherein conductive traces can be prepared on printed circuit board substrate using polymer inks that contain polymer resins and metals, usually silver. Typically heat is used to cure or set the polymers in the inks to form reasonably stable circuit traces. In a manner similar to the Thick Film process, carbon-filled inks can be used to interconnect circuit traces with specific electrical resistances. Carbon prints, known as Polymer Thick Film resistors, can be printed onto traditional copper foil traces, or onto printed Polymer Thick Film conductive circuit traces.
Countless variations of printed circuit boards exist, and many variations of the Polymer Thick Film process also exist. One application of both the printed circuit board process and the Polymer Thick Film process is the Molded Circuit board. Here the process of converting a laminated sheet of material into the proper circuit board dimensions and having all the necessary holes, slots, and shapes are replaced by molding these features into the board. Circuit traces are applied to a board either during or after the molding process. One method for adding the traces was to print them with conductive Polymer Thick Film inks.
In the past the molded board with Polymer Thick Film traces (baking of the Polymer Thick Film ink creates the conductive circuit traces) found limited acceptance for a number of reasons. Printed Polymer Thick Film conductive traces have more resistance than copper foil traces. Also, electronic devices cannot be soldered to most Polymer Thick Film traces. Those electronic devices that were attached to solderable Polymer Thick Film inks did not have good adhesion to the molded substrate after the soldering process. Some Polymer Thick Film conductive inks contain lead which causes environmental concerns and which limits the ability to recycle the materials. Additionally, the molded plastic that could withstand soldering temperatures without warping were the engineering grade materials which are higher quality performing materials. These are more expensive, however, and when used, the cost advantage of the molding process is often lost. Some simple applications of the Molded Board with Polymer Thick Film traces (but without pockets) designed to fit into a connector have been used commercially, but in general commercial production of this type product has been limited.
Printing conductive layers over circuit board traces that are connected to and grounded by a ground plane is a known way to achieve shielding of the traces covered. The circuit traces are first sealed in an insulating layer, and then overprinted with a conductive layer. With this traditional approach it is not possible to shield the components which are attached to the circuit traces, but only the traces themselves.
Lassen""s U.S. Pat. No. 4,602,318 describes achieving high density electronic networks by depositing filaments onto a substrate and encapsulating the filaments to achieve dimensional stability. Filaments are conductive or made conductive by various means. Access to these conductive traces is produced with the use of a high energy beam to cut through and expose the filaments. Lassen claims the use of epoxy resin sheets, and polyimide resin sheets to create his circuitry.
Parker""s U.S. Pat. No. 4,912,844 describes using a heated punch to define grooves and holes in a substrate. The grooves are then filled with solder to create a circuit trace which connects electronic devices. Beaman""s U.S. Pat. No. 5,371,654 describes a three dimensional electronic package with a plurality of assemblies interconnected by aligning the assemblies so they are adjacent, and interconnected by some means such as an elastomeric material, but other than a Polymer Thick Film.
Capote""s U.S. Pat. No. 5,376,403 describes ink formulations which can be used to form circuit traces, but Capote does not describe or claim uses for his ink.
Hiller""s U.S. Pat. No. 5,420,755 places a component in a hole cut into standard circuit board material, but does not claim using molded pockets in circuit boards. The component is attached with a standard solder connection. Placement of the component is in a cut hole and the solder joint is not different from using any common commercial solder joint to connect the electronic devices.
McGinley""s U.S. Pat. Nos. 5,599,595 and 5,688,146 describes how circuit traces can be added to molded plastic to achieve a printed connector assembly. McGinley uses current technology to attach printed Polymer Thick Film conductive traces to the top surface of the Polymer Thick Film traces. McGinley uses current Polymer Thick Film methods to print resistors on the circuitry of the connector.
Marrocco""s U.S. Pat. Nos. 5,646,231, 5,646,232, and 5,654,392 describe the use of rigid rod polymers to form a plastic molded circuit board. No mention is made as to how this is done, nor are any claims made concerning molded pockets in the substrate or attachments of the electrical devices placed in the pockets.
Nakagawa""s U.S. Pat. No. 4,801,489 and Iwasa and Marooka""s U.S. Pat. Nos. 5,066,692 and 4,970,354 describe how printed conductive inks can be used to create shielding properties on printed circuit boards, however all of these patents are for shielding on printed circuit boards. In my invention there is no circuit board, but rather a molded substrate containing inserted components. Also, In my invention the entire package may be shielded, and not just the circuit traces. This is a significant advantage over printed shielding that shields only the traces.
Higgins"" U.S. Pat. No. 5,639,989 describes how shielding of both circuit traces and components mounted on the substrate can be achieved. Higgins patent would require applying an insulating layer over both traces and components and then applying a conductive layer over the insulating layer which connects to a ground plane. This is awkward to achieve since the surface is not planar, and these layers must be applied by spraying, dipping, pad printing, or some other method for applying a uniform thin layer to an irregular surface. In my invention the circuitry and components form a planar surface, and the layers can be easily printed with screen printing or any other common commercial printing process.
The present invention provides a cost effective, highly functional packaged electronic circuit by combining the advantages of molded substrates, Thick Film construction, and Polymer Thick Film technology in a single package. The substrate may also be vacuum formed plastic, and shielding of both circuit traces and components contained in pockets of the molded substrate may be achieved. All of these features are important developments that address the driving forces of the electronic packaging industry, and that is to create smaller and less expensive packaging alternatives. To do this I designed the molded support to accept inserted electronic devices and connecting them with additive circuitry which both adheres to the substrate and interconnects the individual components. In FIG. 1 one variation of this concept is shown. The attached electronic devices may comprise a resistor, a capacitor, an LED, or it can comprise an electro-mechanical device such as a connector pin or an off/on switch, or a bioelectrical functional component. Other simple functional features may also be incorporated into the molded design such as heat sinks, pins that connect front-side circuitry to back-side circuitry, or thermal vias (holes or openings in the board). The electronic device in the pocket may attach on a planar level of the substrate (horizontal plane, two dimensional), or the electronic device in the pocket may attach either below or above the plane of the face of the molded substrate (three dimensional, vertical plane, in the z-axis of the substrate).
Subassemblies, which are smaller circuits complete with their own electronic devices and usually constructed on ceramic substrate, can also be attached in the same manner as electronic devices. This could include ceramic circuitry (complete with active and/or passive components). It could also include ball-grid arrays or chip scale packages. Multichip Modules can also be built up using molded substrates, chips inserted into pockets, and the attachment techniques defined in this document.
The traditional circuit board package begins with a substrate which supports the circuit traces while in the Molded Electronic Package the molded substrate supports both the circuit traces and the electrical devices, and the interconnection of the components is achieved by forming the circuit traces over both the electrical devices and the substrate. Connection can be directly to the electrical devices or can be through vias (small openings or holes) in an insulating layer which covers the electrical devices. In the Molded Electronic Package connection can be directly by the trace or by a second material, such as a solder-paste or a conductive adhesive that is an extension of the trace.
Benefits of this construction are as follows:
1) Since the electronic device is securely held in the pocket by the molded substrate, the electronic device need no longer rely on the adhesion of a Polymer Thick Film conductor ink to the substrate to remain secure in the circuit. Thus, this requirement of attachment or holding of the electronic device is no longer important in the selection of a Polymer Thick Film conductor used to form the circuit traces.
2) Interconnection options are now available that do not require the extreme high temperatures of the soldering process. We therefore have a broader choice of molding material from which to prepare the molded substrate making possible less expensive circuitry.
3) Because electronic devices, especially resistors, can be packaged in pockets in the board in the z-axis rather than mounted to the surface of the board, valuable space is now made available for the attachment of other components. This is a very valuable feature when trying to design more compact circuitry.
4) Because a wide range of materials are available for construction of the supporting molded plastic substrate, the design engineer can take advantage of different dielectric properties such as dielectric constant, voltage breakdown resistance, and loss tangent. This only becomes possible because Molded Electronic Package packaging resolves the problems of heat sensitivity and adhesion properties as discussed above.
5) Because electronic devices such as resistors can now be mounted under the circuit traces in pockets in the molded plastic substrate rather than on top of the traces, one can now route traces to different parts of the circuit without resorting to multi layering the circuitry to avoid crossing the traces.
6) Resistor networks can now be designed below the circuit traces with a higher packaging density than possible with resistors mounted on top of traces, because the connection joint between the trace and the electronic device is no longer also serving as the physical support for the electronic device, and it can therefore be a smaller, more finite joint.
7) Choices of polymer resins are available for the supporting substrate, one being Polyimide, the prefered embodiment, and also are polymers and copolymers of Epoxies, Phenolics, thermoset Polyesters, Syndotactic Polystyrene, Polyethylene Terephthalate, Polybutylene Terephthalate, Polyphenylene Sulfide, Polyamide, Liquid Crystal Polymers, Polyphenylene Oxide, Polycyclo Terethalate and rigid rod polyphenylene. With the broad choice of polymer resins it is now possible and practical to design, build and use circuitry that can be recycled.
8) The preparation of Molded Electronic Package circuitry can be achieved without costly, environmental risky processes, such as the use of lead solders and acids for etching, which are necessary in the current printed circuit board industry.
9) The capitalization required to set up this Molded Electronic Package process is much less than for other printed circuit board factories.
10) Since molded substrates have their physical dimensions defined in the molding process they can be easily stacked in magazines for printing and baking on automated equipment. It is not practical to process traditional circuit board substrates in this way because they must be handled in large sheets to achieve economical conversion to the final size and shape. The adaptability of the Molded Electronic Package to automated handling means its user could set up manufacturing in the country of choice instead of in cheap labor markets as is common in the printed circuit board industry today.
11) Silicon chips can be placed into pockets and attached directly to the Molded Electronic Package board without mounting them first in one of the many carrier alternatives currently used. This not only reduces cost and saves space, but allows easy rework of faulty chips by simply removing the faulty chip from the pocket, inserting a new one, and repeating the printing process which attaches the chip.
12) Since resistors, sub assemblies, and other components can be located in the board and under the circuit traces rather than on the board and over the circuit traces, it is now possible to use an economical print process to seal both the circuitry and the components in dielectric, and overprint the package with a Polymer Thick Film conductive shielding layer.
13) Extremely low cost circuitry can be prepared from vacuum formed plastic. In this process inexpensive sheets of plastic are formed three dimensionally under heat and vacuum pressure. This inexpensive process can be used to form the pockets for inserting components, and other dimensional requirements at the same time.